A plethora of radio frequency (RF) emitters operate in an 83.5 MHz band between 2.4000 and 2.4835 GHz (the 2.4 GHz spectrum) within the globally available, license-free Industrial, Scientific, and Medical (ISM) band. These emitters include wireless; devices having Bluetooth radio transceivers, standardized wireless local area networking (WLAN) applications (e.g., IEEE 802.11), and random noise generators such as microwave ovens and sodium vapor street lamps. As a result, transmissions made by devices operating in the ISM band may be compromised by interference. To reduce such interference, devices having Bluetooth radio transceivers (Bluetooth devices), in particular may employ techniques such as adaptive frequency hopping (AFH), power control, and short data packets.
Bluetooth devices employing AFH “hop” among 79, or optionally fewer, 1 MHz spaced channels in the 83.5 MHz operating band in a pseudo-random fashion at a rate of 1,600 times per second. When another wireless device operating in one or more of these channels is introduced into the same environment as a Bluetooth device, interference among device transmissions may occur. AFH allows the Bluetooth device to adapt to this environment by identifying the channels in which there are sources of interference and excluding them from the list of available “hop” channels. Importantly, a Bluetooth device employs AFH only after it has established a connection with another Bluetooth device whereby packets of data can be sent back and forth between the two devices.
Power control, according to Bluetooth Specification v4.0, which is incorporated herein by reference, is a mandatory feature of all class 1 Bluetooth devices and an optional feature of class 2 and class 3 Bluetooth devices. The radio transceiver of a class 1 Bluetooth device has a maximum output power of 100 mW (20 dBm), the radio transceiver of a class 2 Bluetooth device has a maximum output power of 2.5 mW (4 dBm), and the radio transceiver of a class 3 Bluetooth device has a maximum output power of 1 mW (0 dBm). The power control feature may be used not only to optimize the battery power consumption of a Bluetooth device, but also to reduce the overall interference level among all devices operating in a shared environment and in the 2.4 GHz spectrum.
When a connection has been established between two Bluetooth devices, a receiving Bluetooth device incorporating the power control feature may request an increase or decrease of the operating power of a transmitting Bluetooth device's radio transceiver. The Bluetooth specification requires that increases and decreases in power be controlled in steps of 2 dB to 8 dB. By way of example, if the receiving device determines that reception from the transmitting device is lower than necessary to maintain a satisfactory link, the receiving device will send a request to the transmitting device to increase the power of its radio transceiver. If the radio transceiver of the transmitting device is already operating at its maximum output power, the transmitting device will refuse the request. Conversely, if the receiving device determines that reception from the transmitting device is higher than necessary to maintain a satisfactory link, the receiving device will send a request to the transmitting device to decrease the power of its radio transceiver. If the radio transceiver of the transmitting device is determines that a decrease in power would, for example, cause it to turn off, the transmitting device will refuse the request. However, by honoring the request, the transmitting device will send weaker transmissions that reduce interference.
Short Bluetooth data packets (8 octet minimum up to 27 octets maximum transferred at 1 Mbps) are employed to minimize interference among devices operating in the 2.4 GHz spectrum. If there are many devices in a shared environment operating in this spectrum, such as other Bluetooth or 802.11 radios, microwaves, or cordless phones, then packets may become corrupted while being transmitted over the air. When a receiving device does not acknowledge receiving corrupted packets from a transmitting device, the transmitting device must retransmit in-tact packets to the receiving device. Short data packets exchanged between two connected Bluetooth devices minimize the likelihood of packet corruption, thereby reducing the need for retransmissions. The resulting reduction in packet traffic, in turn, reduces interference.
Noticeably, the aforementioned techniques to reduce interference require an established connection between two Bluetooth devices. However, a Bluetooth device transmits packets that can cause interference even when it is not connected to another device. For example, a first, unconnected Bluetooth device desiring to find out what other Bluetooth-enabled devices are in the area transmits a series of inquiry packets. A second Bluetooth device in the area receiving such an inquiry may reply with Frequency Hop Synchronization (FHS) packets containing all of the information that the first device needs in order to connect to the second device.
In certain settings, terminals having radio transceivers are prevalent in both connected and unconnected states. For example, in a health care setting such as a hospital, bar code reading terminals having Bluetooth radio transceivers are used for purposes including administering proper drugs, treatments, and tests to proper patients at proper times, helping to eliminate counterfeit or expired drugs from being distributed, ensuring that blood of the proper blood type is used in a transfusion, identifying patients, identifying sterilized equipment, keeping track of locations of medical equipment such as heart-testing machines, joint replacements, and surgical staplers, controlling inventory, and monitoring devices used during surgery. Transmissions from these terminals, when connected or unconnected, can interfere with other hospital equipment such as terminal bases, respirators, external pacemakers, mechanical syringe pumps, and kidney dialysis machines, potentially causing such equipment to malfunction. Often times, the radio transceivers of connected or unconnected bar code reading terminals are functioning at an operating power equal to their maximum output power, sending strong transmissions contribute to interference with, for example, such hospital equipment.